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Mystery of the swimming pool

With eyes above water, I can see the bottom of the pool very clearly, almost as if the water wasn't there. Refraction at the water level shifts the grout lines between tiles a little, waves make them dance, but the image of the bottom floor remains entirely sharp.

Once I dive just a few inches below the water level, I still can see the bottom, but it is sudenly entirely blurred as if viewed through the fog. Tiling in particular becomes unidentifiable.

Is this effect of biological, or physical nature?

I wear no goggles. The depth is four metres, the line of vision almost vertical. The water is warm, clean, comfortably low in chlorine. I have some reasons to expect water temperature distribution to be quite uniform on that particular day. I am hoping that this is easily reproducible in perhaps any deep enough public pool.

I am puzzled because I'd expect the optical conditions to degrade, and not improve, with additional distance and an additional medium in between.

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The image becomes blurry because the cornea of your eye (the curved surface of the eye) acts as a lens - the change in refractive index from air to cornea causes the light to start focusing, with "fine tuning" done by the lens inside the eye.

When you open your eyes under water, the cornea is in touch with a medium of almost the same refractive index: water. This means that it cannot focus the light, and the image is blurry.

A picture of an optical model of the eye can be found at the Zemax website

enter image description here

This shows quite clearly that the cornea - the last curved surface on the right - plays a role in the focusing of light. But if the cornea is not in contact with air, the angles of refraction will be all wrong...

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